<!doctype html> 
<html> 
<head> 
    <meta charset="utf-8"> 
    <title></title> 
</head>
<body>
<script type="text/javascript">
	
var a = new String('abc');//'abc'
var b = new String('123');//'123'
b+=b;//b=b+b; '123123'
alert(b);//123
a+=b;//a=a+b  'abc123123'
alert(a);//error

</script>
</body>
</html>